Beta-processed alpha-beta titanium alloys are used to manufacture aerospace hardware such as components of gas turbine engines. These alloys have excellent mechanical properties relative to their weight, at both room temperature and moderate elevated temperatures as high as about 1200° F. The alloys are used to make parts such as fan and compressor disks, blisks, blades, shafts, and engine mounts.
An alpha-beta titanium alloy is an alloy having more titanium than any other element, and which forms predominantly two phases, alpha phase and beta phase, upon heat treatment. In titanium alloys, alpha (α) phase is a hexagonal close packed (HCP) phase thermodynamically stable at lower temperatures, beta (β) phase is a body centered cubic (BCC) phase thermodynamically stable at higher temperatures above a temperature termed the “beta transus” temperature that is a characteristic of the alloy composition, and a mixture of alpha and beta phases is thermodynamically stable at intermediate temperatures. Processing to control the relative amounts and the morphologies of these phases is used to advantage in achieving the desired properties of interest in the alloys.
One approach to preparing articles is to cast the alpha-beta titanium alloy as an ingot, to thereafter thermomechanically work the workpiece from the as-cast ingot form to approximately the final shape and size of the desired article, and to thereafter final machine the article. In beta processing, the workpiece is mechanically worked, typically by forging, at a temperature above the beta-transus temperature, and subsequently heat treated at lower temperatures to reach the desired microstructure. Beta processing is particularly useful for manufacturing large articles, because the strength of the workpiece is reduced above the beta transus temperature, and large workpieces may be mechanically worked more easily in the available metalworking equipment.
In some beta-processed alpha-beta titanium alloys, the ductility of the final article is highly anisotropic and thence strongly dependent upon the angle of the principal loading direction relative to the orientation of the prior beta grain flow that occurs during the beta-phase processing. For example, the tensile ductility measured parallel to the prior beta grain flow direction may be 2-4 times larger than the ductility measured at 45 degrees to the prior beta grain flow direction. This variability in ductility may render the material unsuitable for applications where the article is mechanically loaded in different directions in different portions of the article.
There is a need for an approach to achieving desirable mechanical properties of the beta-processed alpha-beta titanium alloys but also avoiding the anisotropy in ductility and possibly other properties that is associated with some of the beta-processed alpha-beta titanium alloys. The present invention fulfills this need, and further provides related advantages.